The invention is directed to a light-emitting diode comprising a semiconductor layer structure containing a substrate and at least one light-generating layer formed on the substrate, a first electrical contact layer on the substrate and a second electrical contact layer on at least a section of a surface of the semiconductor structure lying opposite the substrate. In particular, the invention is directed to a light-emitting diode wherein the surface lying opposite the substrate comprises a plurality of truncated pyramids on at least one section for improving the light outfeed.
Light-emitting diodes such as semiconductor light-emitting diodes (LED) are particularly distinguished in that the internal conversion efficiency of supplied electrical energy into radiant energy can be very high, definitely greater then 80%, dependent on the material system. The effective light output from the semiconductor crystal, however, is made more difficult by the high discontinuity in the refractive index between the semiconductor material (typically n=3.5) and the surrounding resin casting material (typically n=1.5). The small total reflection angle, which occurs because of the discontinuity at the boundary surface between semiconductor and resin casting material, is approximately 26xc2x0 and leads to the fact that only a fraction of the generated light can be coupled out. A radiation beam that is not emitted in the approximately 26xc2x0 wide output cone remains trapped in the semiconductor crystal in the simple cuboid shape of the LED typically employed in manufacture since its angle relative to the surface normal is also not changed by multiple reflection. The radiation beam, consequently, is lost earlier or later due to absorption, particularly in the region of the contact, the active zone or in the substrate. Particularly given InGaAIP LEDs, the absorbent GaAs substrate represents a particular problem. In conventional LEDs of this species, the rays emitted from the active zone in the direction toward the surface of the LED that lie outside the output cone are lost by absorption in the substrate with high probability.
The approach employed most often in practice to alleviate this problem is to apply a thick semiconductor layer at the upper side of the LED. This enables the partial utilization of the lateral output cone of the emitted light radiation.
U.S. Pat. No. 5,008,718 has disclosed that an electrically conductive GaP layer that is transparent for the emitted light radiation is applied onto the active, light-emitting layers in an AlGaInP LED, mainly for reasons of the lateral spread of the current injected by an electrical contact. The advantageous side-effect of reducing the internal total reflection and enabling the lateral output of the light radiation due to the action of the thick GaP layer is pointed out elsewhere. It is additionally proposed that the GaAs substrate that is opaque for the emitted light radiation be removed by etching and be replaced by at least one transparent substrate layer of a suitable material such as GaP.
U.S. Pat. No. 5,233,204 also proposes the employment of one or more thick and transparent layers in a light-emitting diode. Various configurations are disclosed for the arrangement and plurality of these transparent layers. Among other things, a layer that is arranged under the active, light-generating layer and tapers in the direction toward the substrate and is fashioned funnel-shaped is disclosed in FIG. 10 of this reference.
The solutions that have been hitherto disclosed, however, are either relatively technologically involved or do not produce the desired enhancement of the light output from the light-emitting diode. In particular, the growth of a relatively thick, transparent semiconductor layer is a comparatively time-consuming procedure in the manufacture, since either the metallo-organic vapor phase epitaxy (MOCVD) or the molecular beam epitaxy (MBE) are utilized given these growth methods. The production of a 10-20 xcexcm-thick transparent semiconductor layer is a time-consuming process that lengthens the overall duration of the manufacture of the light-emitting diode in an unacceptable way.
It is thus an object of the present invention to specify a light-emitting diode having a highly effective light output that can be manufactured without additional, complicated or time-consuming manufacturing steps.
This object is achieved by an improvement in a light-emitting diode having a semiconductor layer structure containing a substrate and at least one light-generating layer formed on the substrate, a first electrical contact layer on the substrate and a second electrical contact layer on at least one section of that surface of the semiconductor structure lying opposite the substrate. The improvement is that the surface lying opposite the substrate is structured over at least one section so that it comprises either a plurality of truncated pyramids or a plurality of truncated cones.
In one embodiment of the present invention, the truncated pyramids are implemented three-sided.
In the inventive truncated pyramids, a light beam is diverted into an output cone as a result of multiple reflections. Only beams that proceed steeply toward the upper side of the LED are employed for the output. As a result thereof, long paths in the light-generating layer are avoided. The oblique sidewalls of the truncated pyramids assure that the initially steeply upwardly proceeding rays proceed more flatly with each one of the reflections, so that they are ultimately laterally coupled out from the sidewalls of the truncated pyramids.
Preferably, that section of the light exit-side surface covered with the second electrical contact layer is unstructured. In an exemplary embodiment, the second electrical contact layer given a quadratic or rectangular LED is applied in the form of a cross-structure on the light exit-side surface. This cross-structure is composed of essentially circular connecting pad in the center of the rectangular light exit-side surface and of finger-like terminal surfaces proceeding from the circular circumference in the direction toward the four comers of the chip. The sections lying between these four finger-shaped terminal surfaces are respectively occupied with a plurality of truncated pyramids, these being arranged so that an optimally great plurality thereof can be applied.
There are a number of parameters with respect to the shape of the truncated pyramids that lead to an optimization of the output, as has been investigated in detail given what is referred to as ray-tracing simulations. Optimized parameter ranges for three-sided truncated pyramids are recited below. When the truncated pyramid comprises a base area A and a height h, then V describes the ratio of the root of the base area to the height of the truncated pyramid:
V=Axc2xd/h
Further, the angles of incidence of the three sidewalls of the truncated pyramid xcfx86 and the angles xcex1. xcex2 and xcex3 of the triangular base area are of significance.
The best results are achieved with the following parameter ranges according to the ray-tracing simulations:
xe2x80x830.1xe2x89xa6Vxe2x89xa610
45xc2x0xe2x89xa6xcfx86xe2x89xa688xc2x0
xcex1,xcex2,xcex3 greater than 10xc2x0
Especially good values for the light output derived given V=1, xcfx86=75xc2x0 and an isosceles triangle as the base area having a principal angle xcex8=70xc2x0.
The present invention also comprises the advantage that it is based on a non-local output principle, so that processes for current constriction that are difficult to govern are eliminated. Further, only the upper window need be structured, whereas the active zone is not etched through and is definitely not damaged. The structure is comparatively simple to realize with only one additional lithography process step and subsequent dry etching.
Moreover, truncated pyramids, which have four or more then four sides can also be utilized in the present invention.